Chemotactic activity for neutrophils was demonstrated in exudates collected at lh as well as 4h after the intrapleural injection of carrageenan. The chemotactic activity per mg of lh exudate protein was much greater than that of the 4h exudate. However, the activity of either exudate was inhibited by monospecific antisera to complement component 5 and was destroyed by trypsin and chymotrypsin treatment. Indomethacin (5 mg/kg, i.v.) reduced equally protein content (56% for the 4h exudate and 69% for the lh exudate) and total chemotactic activity (58% for the 4h exudate and 62% for the lh exudate): i.e., chemotactic activity per mg exudate protein was unchanged for these two different aged exudates. These and other data suggest that the exudate chemotactic activity is generated from plasma protein and that indomethacin acts primarily to reduce extravasation of plasma and consequently generation of chemotactic activity. Cells isolated from the pleural cavity (largely macrophages, mast cells, eosinophils and lymphocytes) and incubated with radiolabeled arachidonic acid released substantial amounts of radiolabel upon exposure to carrageenan (1-100 Mug/ml) or antisera to IgE. The time-course and concentration-dependency of this release was compatible with the in vivo actions of these two agents.